Electricity: measuring and testing – Measuring – testing – or sensing electricity – per se – Magnetic saturation
Reexamination Certificate
1999-11-19
2001-11-20
Nguyen, Vinh P. (Department: 2858)
Electricity: measuring and testing
Measuring, testing, or sensing electricity, per se
Magnetic saturation
C324S127000
Reexamination Certificate
active
06320370
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to the field of current measurement, particularly in the context of pulse-width-modulated (PWM) circuits. More specifically, the invention relates to a circuit which provides accurate measurement of current flowing through a load, while maintaining galvanic isolation between the measurement circuit and the load, under both static and dynamic conditions.
Examples of PWM circuits are shown in U.S. Pat. Nos. 5,070,292, 5,081,409, 5,379,209, and 5,365,422. The disclosures of these patents are hereby incorporated by reference. These patents provide examples of circuits in which a series of pulses is used to control electronic switches which selectively connect a power supply to a load. The load can be an electric motor, or a coil used to produce a magnetic field, or some other load.
In PWM circuits of the types described in the above-cited patents, it is often necessary to monitor the current flowing through the load, either for purposes of overcurrent protection, or to control another circuit based on the measured current in the load, or for other reasons. Direct measurement of load current is undesirable because it requires the insertion of an inductance or a resistance into the circuit being monitored. Preferably, the current measurement technique will maintain galvanic isolation, i.e. insuring that no current flows directly between the load and the measuring circuit.
However, in the prior art, there are few techniques for measuring load current in a PWM circuit while maintaining galvanic isolation. While the load can be coupled, through a transformer, to a conventional circuit for current measurement, the accumulation of magnetic flux in the transformer core accentuates the nonlinearity of the transformer and introduces inaccuracy into the final measurement. A solution to this problem is to use a larger transformer, which is less likely to experience core saturation and which therefore provides a greater range over which the transformer response is relatively linear. However, using a larger transformer has the disadvantage of requiring a larger space, and it may also be unacceptably expensive.
In some current measurement circuits, during times in which the sensed load current is changing in response to PWM control signals, the output of the current measurement circuit may not represent the actual load current with the level of accuracy desired. For example, in some current measurement circuits, the load current indicative output can be erroneous by an amount proportional to the rate of change of the load current. The present invention provides accurate measurement of the load current in a PWM circuit under both static and dynamic conditions.
SUMMARY OF THE INVENTION
A circuit for measuring current flowing through a load driven by a pulse width modulation (PWM) circuit in response to PWM control signals, according to the present invention, includes a transformer having primary and secondary windings, with the primary winding connected to the load. Current cancellation circuitry coupled to the secondary winding is adapted to apply a cancellation current to the secondary winding and to provide an output which is indicative of a level of current through the load. An inverter coupled to the current cancellation circuitry output provides an inverted current cancellation circuitry output. First and second switches selectively couple one of the current cancellation circuitry output and the inverted current cancellation circuitry output to a load current output which provides an output indicative of the level of current through the load during both static and dynamic current conditions.
REFERENCES:
patent: 4365422 (1982-12-01), Kawaguchi
patent: 4404623 (1983-09-01), Jourdan
patent: 4542440 (1985-09-01), Cheety et al.
patent: 4584635 (1986-04-01), MacInnis et al.
patent: 4912396 (1990-03-01), Groenenboom
patent: 5070292 (1991-12-01), Goff
patent: 5081409 (1992-01-01), Goff
patent: 5379209 (1995-01-01), Goff
patent: 5629616 (1997-05-01), Weggel
patent: 332737 (1921-02-01), None
patent: 2202338A (1988-09-01), None
MTS Systems Corporation
Nguyen Vinh P.
Westman Champlin & Kelly P.A.
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